Color filters for displays and methods for preparing same

ABSTRACT

The present invention is directed to color filters comprising a transparent substrate and a repeating pattern of colored pixels on the substrate thereby forming the color filter, wherein each of the pixels comprises a plurality of small colorant areas, referred to as a sub-pixel. Processes forming pixels comprising a plurality of sub-pixels exhibit a drastic increase in the formation of usable color filters, resulting in greatly improved yields.

This application is a continuation of U.S. application Ser. No.08/625,580, filed Mar. 28, 1996, now U.S. Pat. No. 5,792,579, which is acontinuation-in-part of U.S. application Ser. No. 08/614,441, filed Mar.12, 1996, now abandoned entitled "Color Filters for Displays and Methodsfor Preparing the Same."

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention is directed to color filters and methods forpreparing color filters. In particular, the present invention isdirected to color filters for use in visual displays and methods forpreparing the same.

2. The Relevant Technology

Color filters are used to produce full color images in visual displays.The three primary colors, red, green and blue used to produce full colorimages in projection displays, flat panel displays and other visualdisplay devices are most commonly provided by color filters.

Generally, color filters consist of a transparent substrate having arepeating pattern of pixels on its surface. Pixels are defined as thesmallest controllable area on visual displays, having the same colorthat are capable of being located and turned "on" and "off" by acomputer display. Each pixel on a color filter is associated with aprimary color, and is arranged with other color pixels in repeatingarrays of red, green and blue triads. Depending upon which pixels lightis passed through, color filters containing pixels of the three primarycolors are capable of producing color images of a wide variety ofcolors.

Although the use of color filters to produce full color images has longbeen known, color filters continue to occupy the highest proportion ofmaterial costs in visual displays, such as flat panel displays. Thisexpense can be attributed to a number of factors, such as color filtermaterial costs and the number of manufacturing steps; however, the highcost of color filters is primarily due to low yields observed in colorfilter manufacturing processes.

Yields in color filter manufacturing process are determined by thenumber of color filters produced that meet industry standards out of thetotal number of filters produced. Panels not meeting industry standardsof approximately 3 to 4 defects per ten inch diagonal must be discardedthereby reducing color filter manufacturing yields and resulting in highproduction cost and low consumer availability. A common defect and majorcontributor to the low yields presently experienced in the industry isthe omission of pixels, a phenomenon commonly referred to as "drop out."When "drop out" occurs, uncolored, unfiltered light passes through thefilter to the eye of an observer as opposed to the color intended to beproduced.

Originally, color filters were prepared by a dyeing, or gelatin processin which a layer of gelatin or other dyeable material formed on theinterior of a transparent substrate was colored using photolithographytechniques. More recently, polyimide systems comprising thermally stabledyes combined with polyimides, have been incorporated intophotolithography processes to improve filter quality. Although colorfilters prepared using photolithography exhibit good resolution andcolor quality, photolithography is labor intensive and results in pooryields. For example, photolithography requires that each colorincorporated onto the filter have a mask, a photoresist, baking andetching steps, and resist removal. To produce a color filter having thethree primary colors, this process must be repeated three times.

Because of the complexity of the photolithography process and thetedious steps that must be repeated for each primary color,photolithography processes have consistently given unsatisfactoryyields, typically on the order of 50%. Even after repairing defectivecolor filters by repeating the photolithography process for omittedpixels, the yield only increases to approximately 70%. Taking intoconsideration the cost of repair and the high percentage of filters thatremain unusable, there has for some time existed a need for a colorfilter manufacturing process having greater yields and consequentlyproducing color filters at lower costs.

More recently, in an attempt to overcome the above-mentioneddeficiencies, a dye diffusion process for making color filters has beenproposed. In this process, a sublimable dye is transferred from a donorsheet containing the dye to a polymeric receiver sheet which becomes thecolor filter. An exemplary dye diffusion process is disclosed in DeBoeret al. U.S. Pat. No. 4,965,242. In DeBoer et al., a dye-donor element isplaced over a dye receiving element, wherein the dye receiving elementcomprises a temporary support having thereon a polymeric alignmentlayer, transparent conducting layer and a polymeric dye receiving layer.Heat is applied to the donor element by radiation energy means, such asa thermal printing head or heat absorption by infrared dyes, causing thedye image to be transferred from the donor sheet to the receiver sheet.Once the transfer has occurred, the dye donor sheet is replaced with aglass support to form the color filter. Other patents disclosing similardye diffusion processes for preparing color filters include U.S. Pat.Nos. 4,962,081, 5,073,534, and 5,242,889.

Here again, although the dye diffusion process simplifies color filtermanufacturing, adequate yields are still not attained. An additionaldrawback that adds to the low yields and increased cost of the colorfilter is that the dye diffusion process is presently limited tosublimable subtractive dyes, namely magenta, yellow and cyan.

Producing primary colors using subtractive dyes requires layers ofmagenta, yellow and cyan to be formed in various combinations to makered, blue and green additive colors. One exception is found in U.S. Pat.No. 5,242,889, issued to Shuttleworth, which discloses a blue sublimabledye. However, because the dye diffusion process must be repeated to formpixels associated with red and green, there exists a greater opportunityfor a defect to occur, resulting in an increased number of unusablecolor filters and, consequently, the continuation of depressed yields.

As expected, the "drop out" rate in color filter manufacturing increasesduring mass production. In addition, as illustrated in FIG. 1, as thepanel size increases, the number of "drop outs" rises dramatically. Forexample, a defect level of one defect per square inch sharply increasesto approximately 50 defects per ten inch diagonal color filter as shownby line 30. A marked decrease can be attained by limiting the defects to0.1 defects per square inch (line 32) and as shown by lines 34 and 36,the number of defects drops sharply when the number of defects isreduced to 1/16 of a defect per square inch (line 34) and 1/160 of adefect per square inch (line 36). Typically, color filters havedimensions of approximately 200 pixels per/inch (wherein 5 mils isequivalent to 127 microns). Panels having as few as three to fourdefects per ten square inch diagonal panel are considered unusableaccording to industry standards. Hence, it is not surprising thattypical yields of current color filter manufacturing processes haveheretofore been on the order of 50% and with repair approximately 70%.

Because of its high color quality, and despite attempts to improve itslow yields, photolithography remains the process of choice in theproduction of color filters for visual displays. This being the case,there remains a need for a manufacturing process that produces colorfilters having good resolution and color quality in high yields.

SUMMARY AND OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provide colorfilters for use in visual displays that are lower in cost than haveheretofore been available.

It is another object of the present invention to provide methods formaking color filters for visual displays that result in higher yieldsthan have heretofore been attained.

It is a further object of the present invention to provide a colorfilter having good color quality and high resolution.

To achieve the foregoing objects, and in accordance with the inventionas embodied and broadly described herein, the present invention isdirected to color filters comprising a transparent substrate and arepeating pattern of colored pixels on the substrate thereby forming thecolor filter, wherein each of the pixels comprises a plurality of smallcolorant areas, hereinafter referred to as sub-pixels.

In accord with the present invention, and contrary to conventionalknowledge, it has been discovered that processes forming pixelscomprising a plurality of sub-pixels exhibit a drastic increase in theformation of usable color filters. When pixels comprising a plurality ofsub-pixels are formed, the omission of less than all of the plurality ofsub-pixels comprising the pixels will not necessarily result in anunusable pixel. Thus, color filters formed in accordance with thepresent invention have greatly improved yields. The repeating pattern ofpixels are composed of a colorant material selected from the groupconsisting of dyes, pigments, inks, and mixtures thereof. Depending onthe process employed, the materials used, and the purpose of the colorfilter, the colorant material used to form the pixels is selected so asto optimize the color quality of the color filter.

Resolution of the color filter formed in accordance with the presentinvention, is optimized by varying the size of the pixels and the numberof sub-pixels comprising the pixel. For instance, reducing the size ofthe pixels in the repeating pattern of pixels and increasing the numberof pixels on a transparent substrate produces a color filter havingincreased resolution.

In a preferred embodiment of the present invention, the color filtercomprises a transparent glass substrate and a repeating pattern of colorpixels on the substrate forming the color filter, wherein each pixel iscomprised of sixteen sub-pixels. With each pixel comprising sixteensub-pixels, the omission of a few sub-pixels constructing the pixel isnot visible to the human eye. Furthermore, it is highly improbable thatmore than three or four sub-pixels will be omitted during pixelformation. Hence, virtually one hundred percent yield will be achievedwhen the pixels formed on the substrate are composed of 16 sub-pixels.Higher than current yields may also be obtained when using pixels havingmore than one but fewer than 16 sub-pixels, although the yield may notbe as high as when using 16 sub-pixels.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the manner in which the above-recited and otheradvantages and objects of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to a specific embodiment thereof which is illustrated in theappended drawings. Understanding that these drawings depict only atypical embodiment of the invention and are not therefore to beconsidered to be limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 is a graph illustrating the prevalence of defects in color filterprocessing.

FIG. 2 is a side view of a donor film 50 and a receiver panel 54 as usedto form color filters in laser ablation transfer processes.

FIG. 3 demonstrates the laser ablation transfer of a sub-pixel from adonor film 50 to a receiver panel 54.

FIG. 4 illustrates an alternative embodiment of the laser ablationprocess wherein the donor film 50 comprises an adhesive layer 74 on thesurface of the colorant-containing layer to aid adherence of a sub-pixelto a receiver panel.

FIG. 5 illustrates color filter construction comprising the combinationof three microsheet layers, each sheet comprising pixels associated witha primary color.

FIG. 6 illustrates a color filter having a primary color pixel arraycomprising red, green and blue pixels.

FIG. 7a is an exploded illustration of a conventional pixel.

FIG. 7b is an exploded illustration of a pixel formed by a laserablation transfer process comprising two sub-pixels.

FIG. 7c is an exploded illustration of a pixel formed by laser ablationtransfer comprising four sub-pixels.

FIG. 7d is an exploded illustration of a pixel formed by laser ablationtransfer comprising six sub-pixels.

FIG. 7e is an exploded illustration of a pixel formed by laser ablationtransfer comprising nine sub-pixels.

FIG. 7f is an exploded illustration of a pixel formed by laser ablationtransfer comprising sixteen sub-pixels.

FIG. 7g is an exploded illustration of a pixel formed by laser ablationtransfer comprising twelve sub-pixels.

FIG. 8 is a view of a black matrix formed on a color filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to color filters and methods forproducing color filters for visual display devices.

Currently, color filter manufacturing technology suffers in that forvarious reasons pixels occasionally fail to form on the transparentsubstrate, resulting in the omission of pixels. This phenomenon isreferred to in the trade as "drop out." When a certain number of pixels,typically three to four pixels for a ten square inch diagonal substrate,do not form on the transparent substrate, the substrate is considered byindustry standards to be unusable and must be discarded. "Drop out"resulting in poor color filter manufacturing yields has been a problemsince the development of color filters for visual displays. These lowyields increase the cost of color filters and consequently contribute tothe high cost of color displays.

It is a feature of the present invention that the yield can be greatlyimproved by forming a plurality of sub-pixels on a transparentsubstrate, so that the plurality of sub-pixels form one pixel. As usedin this application, a sub-pixel is an area of colorant formed onsubstrates used for color filters.

It is readily appreciated that forming each pixel with a plurality ofsub-pixels significantly reduces the effect of "drop out" or omission ofsub-pixels on color filters. It is further appreciated that the effect"drop out" has on the appearance of a pixel is dependent on the numberof sub-pixels comprising the pixel. For example, if the pixel iscomposed of sixteen sub-pixels, the omission of one sub-pixel will havevirtually no effect on the appearance of the pixel to an ordinaryobserver. Even the loss of 3 or 4 sub-pixels per pixel does notsignificantly affect the effectiveness of the pixel. It is still furtherappreciated that unless all the sub-pixels comprising a pixel areomitted, the pixel will be at least partially operable. Hence, thechance that a substrate will be unusable sharply decreases as the numberof sub-pixels comprising the pixels increases.

As used herein, the term "color filter" defines materials used toprovide color in visual display devices including, but not limited to,computers, calculators, televisions, clocks, analog displays inmeasuring devices, instruments, household appliances and audioequipment. Generally, color filters comprise a transparent substrate anda repeating pattern of colored pixels on the substrate. In colorfilters, the pixels are associated with the three primary colors red,green and blue, and are typically arranged in three color triads locatedon the surface of the substrate. Depending on the amount and manner inwhich light is passed through the pixels, full color images can beproduced by a color filter having pixels corresponding to the threeprimary colors.

As illustrated in FIG. 6, color filters 86 have heretofore been composedof a multiplicity of color pixels 88 associated with the primary colors,red, blue, or green. Traditionally, these color pixels have beencomposed of a single small area of one color that is controlled, orturned "on" and "off," electronically. As mentioned above, it is commonin color filter manufacturing processes for a pixel to beunintentionally omitted for one reason or another, resulting in whitelight being emitted through the filter, tainting the color filter andoften rendering it commercially unusable.

As mentioned above, pixels formed according to the present invention canbe composed of a plurality of sub-pixels rather than being made from onecontinuous area of color, and the plurality of sub-pixels forming thepixel can be "turned on and off" collectively. Depending on the size orshape of the pixel desired, the number of sub-pixels can widely vary.The flexibility of forming different shapes and sizes of pixels withgroups of sub-pixels not only allows the user to optimize resolution,but also allows for a greater margin of error when forming the pixels.For example, because the human eye cannot distinguish images below acertain level, a pixel comprising multiple sub-pixels is able to atleast partially conceal defects such as "drop outs".

FIG. 7a schematically illustrates the conventional pixel being composedof one continuous area of dye. If a mistake occurs resulting in afailure to produce a pixel in this area, an observer will see whiteunfiltered light. Typically, three or four mistakes on a ten square inchscreen will render the color filter commercially unusable. Mistakesresulting in the "drop out" of entire color pixels cost industrymillions of dollars each year.

It is a feature of the present invention to reduce the incidence ofunusable color filters by forming each pixel from a plurality ofsub-pixels rather than a single sub-pixel. For example, by referring toFIG. 7b, it will be appreciated that if a pixel having the same area asthe pixel illustrated in FIG. 7a is composed of two sub-pixels, amistake resulting in the absence of one sub-pixel does not have the samedramatic effect as the "drop out" of a pixel, as shown in FIG. 7a. Thus,the failure to produce one of the two sub-pixels forming the pixel ismore acceptable, allowing for a higher percentage of failures to betolerated. In fact, unless all of the sub-pixels comprising a pixel areomitted, the pixel will at least have the partial appearance of thedesired color and, thus, at least be partially operable. Hence, pixelscomprising four sub-pixels decrease the impact of a mistake on theappearance of the color filter as shown in FIG. 7c. It will beappreciated, looking to FIGS. 7d-7f which illustrate pixels composed ofsix, nine and sixteen sub-pixels, that as the number of sub-pixelscomposing a pixel increases, the less impact a mistake will have on theusability of the color filter.

Although pixels composed of sixteen sub-pixels are currently preferred,any number of sub-pixels can be used to form pixels. It is noted thatsub-pixels having a diameter in the range of 10 microns are capable ofbeing formed using present technology, such as laser ablation transfertechnology. This being the case, it is contemplated that a pixel ofaverage size, i.e., 127 microns by 127 microns, can comprise at least144 sub-pixels (12 sub-pixels by 12 sub-pixels).

Pixels may be formed in any shapes and sizes, however, pixels arecommonly formed in either square or rectangular shapes. For example,some color filters have wider horizontal than vertical pixel dimensions,i.e. pixel dimensions of 127 microns horizontally by 95 micronsvertically (FIG. 7g). In this case, assuming the pixel is composed of arow of three vertical sub-pixels and four horizontal sub-pixels, thepixel would be an improvement by a factor of twelve over the "singlearea pixels" commonly used and observable "drop out" would be virtuallynonexistent.

Therefore, the greater the number of sub-pixels used to form a pixel,the greater the margin of error in pixel formation and consequently thegreater the yield. The greater the yield, the more efficient productionis and, in turn, the lower the cost of production. When the presentlypreferred embodiment of sixteen sub-pixels is used to form a pixel, theabsence of one of the sixteen sub-pixels is not distinguishable by thehuman eye. In fact, the absence of two, three, or even four sub-pixelsfrom a pixel composed of sixteen sub-pixels will likely result in littleor no effect to the human eye. It is virtually inconceivable that morethan four sub-pixels will be absent from a given pixel and, therefore,using sixteen sub-pixels to form each pixel results in virtually 100%yield as opposed to the 50% yields currently found in the commonly usedcolor filter processes.

FIG. 1 shows that, as the color filter panel size increases, the numberof panel defects rises dramatically. When the normal defect level of onedefect per square inch is observed, as illustrated by referencecharacter 30, it can be seen that the number of defects quickly rises.Simply by cutting the defect level to 0.1 defect per square inch (32), asubstantial improvement in yield is observed. Where pixels are composedof multiple sub-pixels, the defect level diminishes to virtually noobservable defects per ten square inch color filter panel.

Still further, using a plurality of sub-pixels to form each pixel availsthe producer of color filters the ability to maintain a high color imageresolution as well as an increased yield. When a high yield is theprimary concern, the number of sub-pixels per pixel is increased,whereas when increased resolution is desired, fewer sub-pixels per pixelare desired. Furthermore, the resolution may be increased by decreasingthe pixel size and increasing the number of pixels per visual paneldisplay. Although the optimal number of sub-pixels per pixel will varyfrom filter to filter, it is contemplated that the resolution will notbe the primary concern because the resolution presently achieved withsixteen sub-pixels per pixel adequately meets the ability of the humaneye to discern pixels. It is, however, anticipated that the primaryconcern will be to increase the yield by increasing the number ofsub-pixels per pixel.

Pixels composed of a plurality of sub-pixels may be formed by any methodcapable of producing small areas of colorant that can be groupedtogether to form a pixel. One such method for forming sub-pixels and,correspondingly, pixels, is laser ablation transfer. As will beappreciated by one of ordinary skill in the art, various adaptationshave been included in the laser ablation process to allow for theformation of color filters in high yields. Although laser ablation ispresently the preferred process for forming pixels composed of aplurality of sub-pixels, it is appreciated that other methods, such asthe thermodynamic printing method recited in U.S. Pat. No. 4,792,860issued to Kuehrle, can be used.

Laser ablation transfer (LAT) imaging is a noncontact, intrinsicallyfast method of producing dry color images by direct laser writing. Ingeneral, images are prepared using laser ablation transfer byirradiating a donor film with an optical pulse, typically from anear-infrared laser, which induces a microexplosion in a gas generatinglayer of the donor film causing a high-speed mass transfer of a colorcoating from the donor film to a receiving substrate where the colorantadheres. Accordingly, the receiver substrate is located in closeproximity, preferably in direct contact, to the donor film so that thecolorant has only a small distance to travel.

Because the laser ablation transfer process involves mass transfer, bothnegative images formed on the donor film and positive images located onthe receiver substrate are created simultaneously. An essential elementin the laser ablation process is a film that undergoes laser ablationeven though it has virtually no absorbance in the visible region. Anadvantage of the laser ablation technique is its compatibility with awide variety of strongly colored dyes, pigments and inks. These twoproperties permit precise control over the color and image of the laserablation product formed on receiver substrates.

Presently, two basic donor structures have been developed for laserablation transfer. The first type consists of a polyester transparentsubstrate coated with a thin (typically 1 micron) color coatingconsisting of printing ink, including nitrocellulose and otheradditives, and a near-infrared absorbing dye. Further details of thisprocess are contained in I-Yin Sandy Lee et al., "Dynamics of LaserAblation Transfer Imaging Investigated by Ultrafast Microscopy," JOURNALOF IMAGING SCIENCE AND TECHNOLOGY, Vol. 36, Number 2, March/April 1992,the disclosure of which is hereby incorporated by reference.

The second type of structure similarly consists of a polyester substratecoated with a thin color coating (typically 0.5 microns). However, thesecond type of structure contains an additional layer referred to as adynamic release layer (DRL) consisting of a partially transmitting andabsorbing film, preferably of titanium or metallic aluminum. Inaddition, absorbing dyes can be included in the color coating to lowerthe ablation threshold and to improve the adhesion of the dye to thereceiver sheet. In further contrast to the first type of structure, thecoating in the second type does not melt in the transfer process, but isremoved as a solid flap of color coating. Furthermore, all the heat isconcentrated at the aluminum substrate and aluminum coating interface inthe second embodiment. Thus, a polymer in the color coating layer nextto the aluminum is exposed to the heat, forms a gas, and ejects a pixelof colored material to the receiver sheet where it sticks to form acolor image. Further details on laser ablation transfer can be found inWilliam A. Tolbert et al, "High-Speed Color Imaging by Laser AblationTransfer with a Dynamic Release Layer: Fundamental Mechanisms," JOURNALOF IMAGING SCIENCE AND TECHNOLOGY Vol. 37, Number 4, July/August 1993,the disclosure of which is hereby incorporated by reference.

The laser ablation technique utilized in the present invention to formimages on the color filter, provides a gentle, intrinsically fast methodof manufacturing color filters. Any suitable mechanism that willtransfer the colorant from the donor film to the receiver substrate canbe used, although lasers are preferred for their clean, easy manner oftransferring minute dye and other colorant particles, to form preciseimages. Furthermore, lasers are ideally suited for use in imagingsystems because they can be turned "on" and "off" rapidly, and whenworking in conjunction with an X-Y micro-positioning plot the entirecolor filter can be "digitally" processed.

In a preferred embodiment of the present invention, as shown in FIG. 2,the color filters of the present invention are prepared using a donorfilm 50, having a layer containing a primary colorant 52 and a receiverpanel 54 located in close proximity, preferably in physical contact, tothe donor film. The primary colorant can be any dye, pigment, ink,mixtures thereof, or other material used to color objects. Uponirradiation of the donor film with the appropriate wavelength, thecolorant on the donor film is transferred from the donor film to thereceiver panel. As a result of this mass transfer, a negative image isproduced on the donor film, as a positive image is simultaneouslyproduced on the receiver panel.

The laser ablation technique has proven to provide a simple, delicatemethod of producing pixels or color filters without harsh heating, wetchemicals and tedious mechanical steps. The laser causes a colorant tobe quickly and precisely transferred from a colorant donor to a colorantreceiver without any stress between the colorant donor and the colorantreceiver. Hence, the receiver panel, which becomes the color filter, canbe composed of a material thinner than has heretofore been possible.

In a preferred embodiment of the present invention, the donor film 50 iscomposed of a transparent donor substrate 56, a laser absorption layer58 on the donor substrate, a gas generation layer 60 located on thelaser absorption layer, and a layer containing a primary colorant 52.The receiver panel 54 which receives the colorant and subsequentlybecomes the color filter is composed of a transparent receiver substrate62, preferably glass. In a more preferred embodiment, to aid inadherence of the colorant on the receiver panel 54, a polymeric layer 64is added to the surface of the receiver substrate 62 on the side closestto the donor film 50. In addition, to aid in supporting the receiversubstrate 62, a support 66 can be added to the surface of the receiversubstrate 62 opposing the adherence aiding polymeric layer 64.

FIG. 3, illustrates a preferred method for producing a single area ofcolorant (dot) on a receiver panel that subsequently becomes a colorfilter using laser ablation transfer. First, the donor film 50 and thereceiver panel 54 are oriented in close proximity to one another.Although this distance can vary, it is preferred that the donor film andthe receiver panel be in physical contact with each other. A laser beam68 having a wavelength typically in the range between 760 to 920 nm,preferably 760, 820 or 920 nm is focused to a desired diameter, such asapproximately 30 microns, onto the donor film. It is noted that the sizeof the sub-pixel can be varied, typically by varying the size of thelaser beam. The laser irradiates the donor substrate 56 withsubmicrosecond, near IR laser pulses which are absorbed by the laserabsorption layer 58. The laser absorption layer transforms the radiationinto heat resulting in a localized jump in temperature which iscommunicated from the laser absorption layer 58 to the gas generationlayer 60 causing the gas generation layer 60 to vaporize or partiallydecompose, in a type of microexplosion. The gas generated by thedecomposition of gas generation layer 60 propels the primary colorantlayer 52 to the receiver panel 54 in the form of a dry mass or moltendrop (depending on the structure type of donor film 50) with a forcesufficient to attach the mass to the receiver panel, where itsubstantially instantaneously cools to form a sub-pixel 70. Thesub-pixel 70 formed can be grouped with other sub-pixels to form a pixelor may itself be a pixel. By keeping the receiver panel 54 fixed in oneplace, the donor film 50 can be exchanged in sequence for each of thethree primary colors to produce the color filter.

The gas generation causes the colorant to be expelled from the donorfilm at velocities approaching 331 meters per second. Here again, thesize of sub-pixel 70 generated corresponds to the size of the laser beam68 irradiating donor film 50. Using the above mentioned technique,images can be produced on the receiving panel at a typical frequency of10⁷ pixels/second.

In a preferred embodiment, a coating, preferably a polymeric material,can be deposited onto the surface of the receiver panel 54 by wet or drymethods (i.e., vapor deposition) to aid adherence of the primarycolorant to the receiver panel 54. In still a further preferredembodiment illustrated in FIG. 4, an adhesive layer 74 is added to theouter surface of the donor film 50 on the colorant layer, so that whenthe colorant is thrust from the colorant donor to the receiver panel 54,the colorant will adhere to the receiver panel 54.

The diameter of the sub-pixels produced in laser ablation transfer canbe varied based on the diameter of the laser beam focused onto the donorfilm. Hence, depending on the use of the filter, the sub-pixel size canbe optimized by varying the diameter of the laser beam. Furthermore, incontrast to other color filter manufacturing techniques, laser ablationtransfer can be made to occur faster than any significant transferbetween the irradiated spot and its surroundings by using a shortduration laser pulse, thus limiting the laser ablation transfer effectto the region being irradiated.

In a preferred embodiment of the present invention, the laser ispreferably programmed to strike specific areas, to form desiredpatterns. This is an improvement over current processes for makingfilters, which not only require multiple steps, but for example inphotolithography, require masks to define the boundary areas ofultraviolet light on the resist, which tends to be cost intensive. Forthree color filters having these colors, three masks must be employed,tripling the cost of filter preparation. The present invention replacesthese hardware based processes with a computer that simply directs alaser to form the pattern required.

The radiation used in the laser ablation process can be produced by anysuitable laser with sufficient power to transfer the pixel from thedonor substrate to the receiver sheet. Typically, the energy in thelaser beam should be greater than 200 millijoules/cm². The wavelength ofthe laser used is related to the focal width, which, in turn, willrelate to the number of sub-pixels per inch that can be transferred. Ingeneral terms, as the wavelength decreases from the infrared regionthrough the visible region to the ultraviolet region, the spot size cancorrespondingly be decreased, since the spot size is directly related tothe wavelength. This being the case, the shorter the wavelength, themore sub-pixels can be transferred per linear inch. Semi-conductor diodeinfrared lasers are commonly used in this process because they possesssufficient energy to cause transfer, while at the same time beingrelatively inexpensive and simple to operate.

In a preferred embodiment of the present invention, the laser is a solidstate diode type laser operating at 860 nm, 820 nm or 780 nm. Othersuitable lasers that may be used include, but are not limited to:neon-helium lasers (632.8 nm); argon ion lasers (488 nm or 514 nm);carbon dioxide lasers (10.6 μm); YAG (Yttrium Aluminum Garnet) lasers infrequency double mode: (λ=532 nm); Nd:YLF; (1.05 μm); Ruby (Cr: Al₂ O₃)lasers (694 nm); and ultraviolet emission lasers such as Xe Cl (308 nm)or KrF (249 nm). It is readily understood that the laser used in thelaser ablation process can be programmed to strike a specific area ofthe donor film so that color filters having the desired pattern may beformed.

The donor substrate 56 generally has a thickness between approximately12 μm and approximately 250 μm, preferably 50 μ, and can be any lasertransparent material that allows the laser radiation to travel to thelaser absorption layer 58, provided it is dimensional stable. Suitablesubstrates include polyesters such as polyethylene terephthalate;polyamides; polycarbonate; glassine paper; condenser paper; celluloseesters; fluorine-based polymers; polyethers; polyacetals; polyolefins;and polyimides.

The laser absorption layer 58 located on the surface of the transparentsubstrate is a thin coating of material that can preferably absorbwavelengths in the range between 630 nm and 990 nm, more preferably780-860 nm, and further imparts virtually no color to the resultantreceptor film. Preferably, the laser absorption layer is a metal thatcompletely oxidizes to its corresponding transparent oxide upon exposureto heat, so that the optical properties of the resulting colorsub-pixels are not degraded by any residual laser absorption layer thatmay have followed to the receiver panel. Suitable metals include Al, Ti,Hf; or any alloys of these metals. In addition, other absorbing metals,or antireflective metals such as Al/Ge, Ti/Si or Al/TiO_(x) or evenoptical cavities can be employed, such as opaque metal-dielectricabsorption layer designs. In the preferred embodiment, the laserabsorption layer is titanium.

The laser absorption layer generally has a thickness betweenapproximately 50 and approximately 300 angstroms, and preferably isapproximately 200 angstroms for a single metal. The thickness of thelaser absorption layer is important for a number reasons. First, theabsorption of the layer is directly related to the thickness of thelayer, thus, it is important that the laser absorption layer have athickness sufficient to absorb a sufficient amount of radiation andconsequently provide a sufficient amount of heat to cause the organiclayer to decompose. Furthermore, the laser absorption layer mayaccompany the colorant to the receiving layer, and if the absorptionlayer is of too great a thickness, the amount accompanying the colorantto the receiving layer may adversely affect the color of the resultantpixels. Thicker layers of the absorbing layer may also be undesirablefrom the standpoint of raising the energy threshold for ablation sincemore material must be heated.

The gas generating layer 60 in the donor film of the present inventioncan be any material that will produce a gas bubble in a type ofmicroexplosion at temperatures less than 300° C., preferably less than250° C., to propel a portion of the colorant layer to the surface of thereceiver panel. In a preferred embodiment of the present invention, thegas generating layer is a transparent organic layer, such aspolyethylene carbonate, polyvinyl chloride, ethyl cellulose,nitro-cellulose and polymers such as an acrylate containing aplasticizer having a low boiling point designed to decompose at lowtemperatures to produce a gas bubble sufficient to propel a colorant toa receiver panel. Typical plasticizers include chlorinated paraffins,organo-phosphates, phthalic acid derivatives and glycol derivatives. Lowmolecular components arising from partially uncured radiation or heatcured material, i.e. uncrosslinked components, may also be employed. Thegas generating layer typically has a thickness of between approximately0.1 microns and 5 microns, and is preferably in the range between 1 and2 microns.

The colorant layer 52 preferably comprises a polymer resin and acolorant material. The polymer resin used in the colorant layer can beany suitable polymer, such as acrylates, polyimides, polyurethanes orwaxes, that are capable of maintaining their integrity during themanufacturing process steps. As mentioned above, the colorant can be anysuitable dye, interference and non-interference pigment, ink or mixturethereof. Moreover, both the colorant and polymer should be light stableto produce quality color filters that do not degrade in time due tolight exposure.

The wide variety of colored dyes, interference and non-interference,inorganic or organic based pigments, colored waxes, and inks that can beused in the laser ablation transfer process, makes it possible toproduce high-resolution color images, especially when exact selection iscritical. Transferable dyes include those based on azo, anthraquinone,quinophthalone and methine chemistries. Furthermore, the laser cantransfer mass colorants (sub-pixels) as small as 10 microns in diameter.Therefore, the number and size of the pixels can be varied to includelarger or smaller pixels, and more or fewer pixels per display, tooptimize the resolution of the display. By optimizing pixel color andsize, using laser ablation transfer, a resolution comparable to that ofphotolithography (10 to 20 microns) can be achieved.

The receiver panel 54 can be composed of any non-birefringent materialthat suitably receives and retains the colorant propelled from the donorfilm such as a transparent substrate having a thickness of about 25microns to about 250 microns, preferably a glass sheet having athickness between 25 and 50 microns. Until the present invention, glassof this thickness has not been feasible because of the strain and stressexerted on the receiver panel during conventional manufacturingprocesses. However, because there is no stress between the donor filmand the receiver panel in laser ablation transfer techniques, delicatematerials such as micro-thin glass can be employed in the production ofcolor filters. The use of layer 66 aids in maintaining the integrity ofthe thin glass color filters. The thickness or lack thereof, of thereceiver allows for a thinner, lighter color filter product thatimproves image quality. These qualities are especially useful in compactcolor displays, such as lap top computers.

In a preferred embodiment, the receiver panel further comprises anadherence aiding layer 64, located on the transparent substrate nearestthe donor film. Any material that will aid adherence of thecolorant/polymer material transfer can be used, suitable materialsinclude, but are not limited to: polyamides, melamines, acrylics,polyurethanes or polyesters and other high energy surface materials.

In an alternative embodiment, an ultraviolet, visible light or heatactivated adhesive is used to selectively remove sub-pixels from a donorfilm. In the present embodiment, the donor film and the receiver panelare oriented so that the heat activated adhesive layer 74 of the donorfilm and an adherence aiding layer 64 on the receiver panel are incontact. Using the heat activated system, a laser beam 68 heats up aselected area, i.e., 30 microns in diameter, of an absorption layer 58which causes the adhesive layer 74 to activate and bond the colorantlayer 52 to the adherence aiding polymeric layer 64 of the receiverpanel 54. Instead of functioning as a gas generating layer, layer 60 mayfunction as a release layer allowing the color sub-pixel to transfer tothe receiver panel. In this case, layer 60 may include compositions ofcellulose, fluoropolymer, silicone or wax based chemistry. As with otherembodiments of the laser ablation transfer process, the size of theheated area can be varied by adjusting the diameter of the laser beamradiation. Subsequently, upon removal of the receiver panel 54 and thedonor film 50, the sub-pixels produced by the laser ablation process arereleased to the panel. Here again, by keeping the receiver panel 54fixed in one place, the donor film 50 can be exchanged in sequence foreach of the three primary colors to produce the color filter.

Any suitable adhesive material that does not adversely affect theoptical quality or color of the filter can be used for the adhesivelayer on the donor film, including: cyanoacrylates, nitrile-phenolicelastomers, epoxides, polyesters, modified acrylics, polyurethanes andpolymethyl methacrylate or other materials that will bond to thereceiver layer surface (64). Alternatively, the adhesive layer cancontain infrared absorbing, but visibly transparent materials, such asinfrared absorbing dyes, to aid the absorption of infrared radiationused to heat the adhesive layer 74.

In a further related embodiment of the present invention, laser ablationtransfer can be used to form color filters wherein a filter substratecomprising a transparent substrate and a colorant layer on thetransparent substrate becomes the color filter. The colorant layercomprises a primary dye, pigment, ink or other colorant and an absorbingmaterial, as disclosed above, for absorbing the laser radiation. Asdiscussed above, laser ablation transfer involves mass transferresulting in a positive image being formed on a receiver and a negativeimage being formed on the donor. Accordingly, it is possible to ablateall the colorant except the desired sub-pixel arrangement using a filtersubstrate having a colorant layer on one side of a thin transparentsubstrate. The present process is repeated three times, one for eachprimary color to produce three separate filter panels 81, 82 and 83which are laminated, aligned and bonded together to form a color filteras illustrated in FIG. 5.

Alternatively, due to the precise nature of the laser ablationtechnique, one filter substrate can be coated on both sides, one sidewith one primary color and the other side with another primary color.Subsequently, sub-pixels corresponding to the two primary colors can beformed by removing everything but the sub-pixel arrangement by laserablation, i.e. blue sub-pixels on one side of the sheet and redsub-pixels on the opposing side. The filter substrate comprising colorson both its front and back surfaces is combined with another filtersubstrate comprising sub-pixels corresponding to the third primarycolor. The two filter substrates are laminated, aligned and bondedtogether to form the color filter. This process reduces the thicknessand the weight of the filter. Whether two or three donor films are used,the thinness of the transparent substrates used prevents any substantialparallax from occurring.

In still a further embodiment, as illustrated in FIG. 8, the blackmatrix grid 84 surrounding the color pixels can be formed using laserablation transfer. FIG. 8 illustrates a color pixel comprisingsub-pixels 88, surrounded by a black matrix grid 84. The role of theblack matrix is to block the thin film transistor from the light,separate the color patches from each other, to define the areas of thepixels, and to improve the contrast ratio of the color filter bypreventing color contamination or light flare. The laser ablationtransfer process is used in the same manner as described above for theformation of sub-pixels on the receiver panel: (i) the donor film isirradiated with a suitable wavelength; (ii) the absorbing layer heats upcausing the gas generating layer to decompose producing a gas bubble;(iii) the gas produced by the gas generating layer propels a mass ofblack colorant precisely to the desired location on the receiversubstrate to form a black matrix 84 surrounding and slightly overlappingthe color pixels.

Alternatively, as mentioned above, pixels composed of a plurality ofsub-pixels can be formed by the thermodynamic printing process disclosedin U.S. 4,792,860, the disclosure of which is hereby incorporated byreference. Accordingly, the pixels of the present invention are preparedby forming a printing means having a printing surface and a multiplicityof independently electrical chargeable capacitor microcells adjacent tothe printing surface. Selected microcells are activated in accordancewith incoming data so that the activated microcells are geometricallyrelated to the sub-pixels printed. Electrical charges are deposited onthe microcells selected for activation at controlled variable colombiccharge levels to create, at the printing surface, localized electricfields of various strengths that are proportional to the print densitiesdesired for the related data in the sub-pixels printed. The printingsurface is then contacted with a voltage sensitive ink, whereby the ink,under the influence of said fields, is deposited on the printing surfaceonly at the locations of the microcells selected for activation, withthe thickness of each of the ink deposits being proportional to thestrength of the field at that microcell thereby to form a uniform or avariable thickness ink pattern on the printing surface. The sub-pixelsare subsequently transferred to the printing medium to form a pixel. Byadjusting the thickness of the print layer, compensation for absorptionstrengths of the colorants is easily accomplished.

Using the present method, sub-pixels of any size ranging from as smallas 10-30 microns can be formed. The printing medium onto which thesub-pixels are transferred can be any suitable substrate, includingglass and plastic substrates, having a wide range of thicknesses. Thesesubstrates will become the color filters. Furthermore, the thermodynamicprinting method is capable of printing fully variable color filtersutilizing pixels comprising a plurality of sub-pixels, on demand atspeeds in excess of 1 sq. meter/sec.

In addition, the present thermodynamic printing method can be used toform the black matrix 84 illustrated in FIG. 8. The thermodynamicprinting process is performed in the same manner as described above,except, a black colorant is deposited onto the printing medium to form ablack matrix surrounding, and typically slightly overlapping with, thecolor pixels.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A color filter for use in visual displays comprising:(a) atransparent glass substrate having a thickness of about 25 microns toabout 250 microns; and (b) a repeating pattern of pixels on saidsubstrate forming a color filter, each of said pixels being formed of aplurality of individual sub-pixels, each of said sub-pixels beingseparately and individually formed on said substrate separately fromother sub-pixels so that a failure in the formation of one sub-pixelwill not result in the failure to form adjacent sub-pixels, and so thatan omission from the surface of said substrate of less than all of theplurality of sub-pixels comprising said pixel will not necessarilyresult in an unusable pixel.
 2. A color filter for use in visualdisplays as recited in claim 1, wherein said transparent glass substratehas a thickness between about 25 microns and about 50 microns.
 3. Acolor filter for use in visual displays as recited in claim 1, whereinsaid transparent glass substrate has a thickness of approximately 25microns.
 4. A color filter for use in visual displays as recited inclaim 1, wherein said transparent glass substrate further comprises alayer for aiding adherence of said pixels to said substrate.
 5. A colorfilter for use in visual displays as recited in claim 1, wherein saidtransparent glass substrate further comprises a support layer.
 6. Acolor filter for use in visual displays as recited in claim 1, whereinsaid transparent glass substrate further comprises a support layer and alayer for aiding adherence of said pixels to said substrate.
 7. A colorfilter for use in visual displays as recited in claim 1, wherein saidsub-pixels are between 10 microns and 50 microns in size.
 8. A colorfilter for use in visual displays as recited in claim 1, wherein saidsub-pixels are approximately 30 microns in size.
 9. A color filter foruse in visual displays as recited in claim 1, wherein said plurality ofsub-pixels forming said pixel is a number of sub-pixels between twosub-pixels and sixteen sub-pixels.
 10. A color filter for use in visualdisplays as recited in claim 1, wherein said pixel is rectangular inshape.
 11. A color filter for use in visual displays as recited in claim1, wherein said pixel is square in shape.
 12. A method for preparing acolor filter for use in visual displays, said method comprising thesteps of:(a) orienting a donor film and a receiver panel in closeproximity, wherein said donor film comprises a transparent donorsubstrate having a first side and a second side, an absorption layer onsaid second side of said donor substrate and a colorant layer on saidabsorption layer, and said receiver panel comprises a transparent glassreceiver substrate having a thickness of about 25 microns to about 250microns; (b) irradiating said first side of said donor film with anirradiating means focused on the donor film, so that said absorptionlayer absorbs said radiation and causes a portion of said colorant layerto be transferred to said receiver panel to form a plurality of separateand individual sub-pixels, the plurality of sub-pixels forming a pixelthat is likely to be usable even in the event that one or more of thesub-pixels are improperly formed; and (c) repeating said irradiatingstep so that a repeating pattern of pixels is formed on said receiverpanel to form a color filter.
 13. A method for preparing a color filterfor use in visual displays as recited in claim 12, further comprisingrepeating said process so that said color filter comprises a repeatingpattern of red, green and blue pixels.
 14. A method for preparing acolor filter for use in visual displays as recited in claim 12, whereinsaid irradiating means is a diode laser, and wherein said diode laser ispulsed so as to form each of the plurality of separate and individualsub-pixels.
 15. A method for preparing a color filter for use in visualdisplays as recited in claim 12, wherein said absorption layer comprisesa thin layer of a metal that oxidizes to a transparent oxide uponheating by irradiation with a laser.
 16. A method for preparing a colorfilter for use in visual displays as recited in claim 15, wherein saidthin layer of metal is selected from the group consisting of titanium,aluminum, hafnium, and alloys thereof.
 17. A method for preparing acolor filter for use in visual displays as recited in claim 15, whereinsaid thin layer of metal is titanium.
 18. A method for preparing a colorfilter for use in visual displays as recited in claim 12, wherein saidreceiver panel further comprises an adherence aiding layer on a surfaceof said receiver panel facing said donor film.
 19. A method forpreparing a color filter for use in visual displays as recited in claim18, wherein said donor film further comprises an adhesive layer on saidcolorant layer.
 20. A method for preparing a color filter for use invisual displays as recited in claim 19, wherein said adhesive layer isselected from the group consisting of a heat-activated adhesive, anultraviolet light-activated adhesive, and a visible light-activatedadhesive.
 21. A method for preparing a color filter for use in visualdisplays as recited in claim 12, wherein said absorption layer comprisesa material selected from the group consisting Al/Ge, Ti/Si, Al/TiO_(x),and combinations thereof.
 22. A method for preparing a color filter foruse in visual displays, said method comprising the steps of:(a)providing a donor film comprising a transparent donor substrate having afirst side and a second side, an absorption layer on said second side ofsaid donor substrate, a colorant layer on said absorption layer, and anadhesive layer on said colorant layer; (b) providing a receiver panelhaving a first side and a second side, and a polymeric adherence aidinglayer on said first side, said receiver panel comprising a transparentglass receiver substrate having a thickness of about 25 microns to about250 microns; (c) orienting said donor film and said receiver panel suchthat said adhesive layer of said donor film is in contact with saidadherence aiding layer of said receiver panel; (d) irradiating saidfirst side of said donor film with a laser focused on the donor film sothat said absorption layer absorbs radiation and causes the adhesivelayer to activate and bond a portion of said colorant layer and saidadhesive layer to said receiver panel to form a plurality of separateand individual sub-pixels, the plurality of sub-pixels forming a pixel;(e) repeating said irradiating step so that a repeating pattern ofpixels is formed on said receiver panel; and (f) separating the donorfilm and the receiver panel to release the sub-pixels to the receiverpanel to form a color filter.
 23. A method for preparing a color filterfor use in visual displays as recited in claim 22, wherein the thicknessof the transparent receiver substrate is about 25 microns to about 50microns.
 24. A method for preparing a color filter for use in visualdisplays as recited in claim 22, wherein said absorption layer comprisesa material selected from the group consisting of Al/Ge, Ti/Si,Al/TiO_(x), and combinations thereof.